In recent years, there is a strong need for preventing air pollution due to exhaust gas, promoting the use of fossil fuel alternative energy, and preventing global warming due to carbon dioxide. A technology for electric vehicles, hybrid vehicles or the like is being developed. Further, the market for innovative terminal devices such as electronic devices, in particular, smartphones is remarkably growing. In relation to these technological innovations, an electrochemical capacitor is being developed as a new power storage device for the purpose of drive power assist, recovery and effective use of energy.
The electrochemical capacitor is a power storage device characterized by much higher speed charging and discharging characteristics and higher cycling characteristics than a secondary battery. There are two major types of electrochemical capacitor including an electric double layer capacitor and a lithium ion capacitor. The electric double layer capacitor usually employs an electrode containing activated carbon as a positive electrode and a negative electrode. Physical adsorption and desorption of electrolyte ions to the activated carbon causes charging and discharging. This does not involve a chemical reaction, so that the electric double layer capacitor is less susceptible to deterioration. The electric double layer capacitor is thus characterized by excellent cycling characteristics. On the other hand, the lithium ion capacitor usually employs as a positive electrode an electrode containing the same activated carbon as that used in the electric double layer capacitor, and as a negative electrode the same lithium-occluding carbon material as that used for the negative electrode in a lithium ion battery. Charging and discharging is performed by absorbing and desorbing electrolyte ions at the positive electrode and occluding and releasing lithium ions at the negative electrode.
Conventionally, activated carbon in a particulate form or in a powder form which has been used as an electrode material for an electrochemical capacitor is easily flocculated in the electrode and has limited contact area with an electrolytic solution. Therefore, it has been difficult to provide higher electrostatic capacitance for the electrochemical capacitor. Moreover, flocculation of activated carbon increases flow resistance of the electrolytic solution, which adversely affects the further improvement of the high-speed charging and discharging characteristics of the electrochemical capacitor. Even in the case where the activated carbon has many pores formed on its surface to have a high specific surface area, these pores are not in communication, so that there has been a problem in that the surfaces inside the flocculated activated carbons are not in use. It has been therefore highly desirable to develop a porous carbon material capable of efficiently utilizing its surface.
For example, Patent Document 1 discloses a capacitor having an electrode containing a porous carbon material, in which pores and carbonaceous walls that serve as external walls of the pores are in a three-dimensional network structure.